In light of these statistics, the U.S. Preventive Services Task Forces (USPSTF) recently updated its colorectal cancer screening guidelines, expanding its recommendations to include adults aged 45 to 75. Previously, in their 2016 guidance, screening was only recommended for adults aged 50 to 75. (For older adults, screening may be done but the benefits—in terms of increased life expectancy—may be minimal.)
This article looks at several tests approved by the USPSTF for colon cancer screening, and how they are used. They include direct visualization tests like colonoscopy, as well as less-invasive stool-sample tests for people with lower risk. The screening frequency depends on the test, or combination of tests, that are used.
Stool Tests
One of the characteristic symptoms of colon cancer is hematochezia (blood in stool). Stool-based screening tests are used to check for evidence of blood in a stool sample, which can often go unseen in early-stage disease. Newer stool tests also check for cancer biomarkers (a substance or process suggestive of cancer).
Earlier screening may help people with a family history of the disease or certain inflammatory bowel diseases that increase risk. Speak with your healthcare provider to know when your screening should begin.
Hematochezia can occur for many reasons and, as such, any positive test result only suggests that cancer is possible and that further investigation is needed.
The different stool-based tests also have varying degrees of sensitivity and specificity and may be less accurate in the early stages.
Guaiac-Based Fecal Occult Blood Test (gFOBT)
Occult blood is blood you cannot see with the eye. The guaiac-based fecal occult blood test (gFOBT) involves the placement of a small sample of stool on a piece of paper infused with guaiac (an organic compound derived from the resin of the Guaiacum tree).
Hydroxide peroxide is then applied. If blood is present, the paper will turn blue within seconds.
Although convenient, the gFOBT has a sensitivity of only 31% (meaning that it can correctly confirm a positive result in 31% cases) and a specificity of 87% (meaning that it can correctly confirm a negative result in 87% of cases).
In its latest guidelines, the ACG notes the use of gFOBT testing has been largely replaced by other, more accurate tests that do not require dietary modifications or medication restrictions.
Fecal Immunochemical Test (FIT)
The fecal immunochemical test (FIT) is a stool-based test that uses a specific antibody to check stool for hemoglobin (a protein in red blood cells that carries oxygen molecules). The FIT is far more accurate and cost-effective than the gFOBT and is associated with improve screening rates in underserved populations.
Despite these benefits, the FIT has its limitations. Hemoglobin degrades at high temperatures and improper storage of stool samples can increase the risk of false-negative readings. Studies have shown that, for this reason alone, the FIT has a lower sensitivity in the summer months compared to winter months.
Moreover, the FIT tends to be less accurate in the early stages of colon cancer. A 2020 review of studies in the American Journal of Gastroenterology concluded that the FIT has a sensitivity of 73% with stage 1 colon cancer, increasing to between 79% and 83% for stage 2, stage 3, and stage 4 disease.
FIT-DNA Test
The FIT-DNA test (also known as a multi-target stool DNA test) is a newer version of the FIT and, in addition to using antibodies to check for hemoglobin, can also detect 10 molecular biomarkers for colon cancer (including KRAS mutations common to many types of cancer).
The first FIT-DNA test approved by the U.S. Food and Drug Administration (FDA) in 2014 is marketed under the brand name Cologuard.
Studies have shown that Cologuard has higher sensitivity in detecting colon cancer than even colonoscopy and can correctly diagnose the disease in 92% of asymptomatic people at average risk of the disease.
With that said, Cologuard is far from perfect. In people with large advanced polyps—those measuring over 10 millimeters in diameter—the test is able to detect fewer than half (42%). This limits Cologuard’s ability to detect precancerous polyps before they become malignant.
Moreover, Cologuard has lower specificity than traditional FITs (87% to 90%), increasing the risk of false-positive results.
Home-Based Stool Tests
Both gFOBT and FIT are available as home self-test kits without a prescription. You can also request a prescription from your healthcare provider to purchase an at-home Cologuard test.
These home-based tests may be an option for those who might otherwise avoid screening but are only appropriate for those who are at low risk of colon cancer. This includes people who do not have:
A personal colorectal cancer or certain types of polyps A family history of colorectal cancer A personal history of inflammatory bowel disease (IBD) Prior exposure to abdominal or pelvic radiation therapy
The home-based tests are readily available for purchase online, although some require you to confirm that you are age 45 or over.
With the gFOBT, test results can be rendered immediately at home. With FIT assays and Cologuard, the stool samples are mailed to the lab and the results are returned electronically or by post, usually within several days. The cost of the home kits may or may not be covered by insurance.
As convenient as the tests are, there are drawbacks to their use. Many of the home-based products are not well-studied or standardized. Moreover, user error is common and can undermine the test results.
Colonoscopy
Colonoscopy is a minimally invasive endoscopic procedure used to view the entire length of the colon (also known as the large intestine). The procedure involves a flexible fiber-optic scope, called a colonoscope, that is inserted into the rectum to view live images of the colon on a video monitor.
Colonoscopy allows the healthcare provider to obtain tissue samples or remove polyps with specialized attachments fed through the neck of the scope.
Colonoscopy is typically performed under intravenous sedation to induce “twilight sleep.” Bowel preparation is required a day before the procedure.
Overall, colonoscopy has a sensitivity of roughly 74%, but, unlike stool-based tests, are far more likely to spot high-grade polyps and remove them before they become cancerous. Where colonoscopy sometimes falls short is in its ability to navigate many of the twists and turns in the large intestine.
Women, for example, tend to have longer colons packed into smaller abdominal cavities. As a result, it is often difficult to reach the cecum at the furthest end of the colon (particularly in young, slender women). It can also be difficult to navigate the sharp bend in the colon, called the hepatic flexure, in tall men or people with obesity.
Even so, colonoscopy remains the gold standard for colon cancer screening, according to the American Society of Gastrointestinal Endoscopy (ASGE).
Flexible Sigmoidoscopy
Flexible sigmoidoscopy is a mini-version of a colonoscopy used to visualize the portion of the bowel closest to the anus, called the sigmoid colon.
The procedure involves a flexible scope, called a sigmoidoscope, which is inserted into the rectum to view only the lower third of the colon. (There are also rigid sigmoidoscopes, but these are rarely used for screening purposes.)
One of the main differences between sigmoidoscopy and colonoscopy—and the one most often cited by people undergoing the procedure—is that bowel preparation tends to be less complicated with sigmoidoscopy. In addition, anesthesia is often not required.
That is not to suggest that sigmoidoscopy is appropriate for everyone. Men, for instance, are more likely to develop cancer in the rectum or sigmoid colon and may benefit from the procedure. By contrast, women are more likely to get cancer in the upper two-thirds of the colon (mainly the cecum and ascending colon) and may not.
Factors like these translate to a lower overall efficacy when compared to colonoscopy. According to a 2019 study published in the International Journal of Colorectal Diseases, sigmoidoscopy was only about half as accurate in detecting cancer with a sensitivity of around 35%.
Virtual Colonoscopy
Virtual colonoscopy (VC), also known as CT colonography, is a non-invasive imaging study used to visualize the colon. It involves computed tomography (CT), an imaging technique in which multiple X-rays are composited to create three-dimensional “slices” of internal structures.
VC doesn’t involve the extensive bowel prep of endoscopic procedures (generally a laxative a day before the procedure and a suppository to clear any remaining residue) and doesn’t require sedation or anesthesia of any sort.
VC has yet to play a major role in colon cancer screening, but advances in technology and increasing acceptance among healthcare providers and the public may one day change that. With that said, the benefits of VC are counterbalanced by a number of risks and limitations.
According to 2018 guidance from the American College of Radiologists (ACR), the overall sensitivity and specificity of VC screening are high (90% and 86%, respectively), but the sensitivity drops to 78% when polyps are 6 millimeters or less. Real-world studies suggest that the sensitivity may be closer to 44% when polyps are small.
VC can also miss smaller sessile (flat) polyps. Although these polyps are at no greater or lesser risk of turning cancerous, the fact that they are missed allows them to go unnoticed until such time as they become malignant.
A 2016 study in the journal Radiology reported that more than half of malignant polyps detected in follow-up VC screenings were sessile polyps that had gone unnoticed during the initial VC screening.
At present, VC may be considered for people who decline colonoscopy. For its part, the USPSTF acknowledges its use but has yet to formally endorse VC for colon cancer screening.
A Word From Verywell
When it comes to colon cancer screening, there is not one option that is inherently “right.” There are pros and cons to each, as well as personal and financial considerations that need to be taken into account (including the resources available for testing and follow-up).
Speak to your healthcare provider to discuss which options are most appropriate for you based on your level of risk. People who have IBD, a personal or family history of colon cancer, or a prior history of abdominal or pelvic radiation may be well served to consider colonoscopy—the procedure of which is rarely as problematic as people think.
